Magnetographic printing machines are known which, in response to signals received from a control unit, enable the formation of images, for example images of characters, on a printing medium, generally a strip or sheet of paper. In these printing machines, which are analogous to those described and shown in the French patent application published as No. 2.305.764, corresponding to U.S. Pat. No. 3,945,343, the printing of the images is attained by first forming a latent magnetic image on the surface of a magnetic recording element, which generally takes the form of a rotating drum or an endless belt, based on received signals. This latent image comprises a group of magnetized zones of very small dimensions which are practically punctiform and are conventionally known as magnetized points. This latent image is then developed, that is, made visible, with the aid of a developer in powdered form, which comprises magnetic particles and pigments coated with a thermoplastic resin and hence is attracted only by the magnetic zones in the recording element, thereby forming an image in powder on the surface of this element. This image in powder is then transferred onto the printing medium.
For certain specific applications, it may be desirable for the image thus formed on the printing medium to appear in two different colors. In a known method, which has been described particularly in French Pat. No. 1.053.634, an image is printed in colors on the printing medium by, first, forming a latent magnetic image on the recording element, corresponding to the portions having the same color in the image that is to be printed, then developing this latent image by using a developer having this color, next transferring the image in powder that is thus obtained onto the printing medium, and finally repeating this operation as many times as there are colors in the image that is to be printed. Such a method has the disadvantage, however, that it takes a particularly long time to perform. Furthermore, despite all the care taken in lining up the different images in powder when they are transferred to the printing medium, it is practically impossible to avoid some shifting, however slight, among the various portions of the image printed in this way. This shifting disadvantageously impairs the sharpness of the image finally formed on the printing medium.
To overcome these disadvantages, a magnetographic printing method has been proposed. This method is described in U.S. Pat. No. 3,965,478, and comprises forming a plurality of magnetized zones on the surface of the recording element, which taken together constitute a latent magnetic image; each of these zones is obtained by exciting a magnetic recording head using an electric current the frequency of which is selected as a function of the color this zone is intended to provide when it is developed; the dimensions and the force of magnetic attraction of this zone are also determined by the value of the frequency used. In this method, developing the latent image formed on the recording element is attained by means of a single developer, containing particles of various colors and various sizes; the particles that are of the same size are also the same color. When the latent image is developed, the particles having a particular color (and hence a given color) are preferentially attracted by the magnetized zones having dimensions responding to a given force of attraction, such that after developing, each magnetized zone is covered by particles the color of which corresponds to the frequency that has been used to form this magnetized zone.
To perform a method of this kind, however, a developer must be used in which the particles, which are of different colors and different sizes, must be carefully calibrated. The particles of the same color must be very rigorously assured to have the same size. Furthermore, these particles must be conditioned such that they will not clump together, or else errors in color tonality will occur when the latent magnetic image is developed. Under these circumstances, it takes a particularly long time and it is particularly delicate and expensive to manufacture such a developer. On the other hand, since the magnetized zones formed on the recording element are not all the same size, depending on the color assigned to them, the images or portions of the image in which the color corresponds to large magnetized zones are poorer in definition, or fineness, than those in which the color corresponds to small magnetized zones. Finally, while the small-sized magnetized zones, when they are developed, are capable of attracting only the smallest particles in the developer, it is impossible to prevent the large-sized magnetized zones from attracting not only the large particles in the developer but also the smaller particlees, which naturally disadvantageously alters the colors.
Various magnetographic printing methods have been proposed for overcoming these problems, and reference may be made to French patent applications filed by the assignee of the present invention on Dec. 23, 1981 and published as Nos. 2.518.770, 2.518.771, 2.518.772 and 2.518.773, corresponding to U.S. Pat. Nos. 4,449,132; 4,449,130; 4,449,131; and 4,449,133, respectively. These methods enable high-quality images in color to be obtained on the printing medium within a relatively reduced length of time. In these methods, as many powdered developers are used as there are different colors in the image that is to be printed. The developers used in these methods all comprise high-retentivity particles coated with a thermoplastic resin containing a pigment, and they all have the characteristic that except for their color they have the same physical properties, and in particular the same granulometric state, the same coercive field, the same induction to saturation, the same density and the same melting point. It will be remembered that a high-retentivity material is a material which can be magnetized in a permanent manner below the Curie point, or magnetic transition temperature, while a low-retentivity material is a magnetic material which, when it is subjected to the action of an external magnetic field, retains practically no magnetization when it is withdrawn from this field.